The present invention relates to an automatic adjusting device in a disc brake for adjusting a clearance between a brake disc and a friction pad so as to form a given clearance therebetween. The present invention has its primary object to automatically obtain a sufficient amount of backward movement of a piston for urging a friction pad, when the brake is released, so that a dragging phenomenon of the friction pad to the brake disc is eliminated even if the great lateral oscillation of the rotating brake disc is encountered thereby preventing the wear of the friction pad resulting from said phenomenon. The invention has another object to make use of constituent members of the aforementioned device so that air within the hydraulic cylinder may be removed to simplify the construction.